Nanoscale optics is the study of optical interactions with matter structured into units of subwavelength (for visible light) dimensions. Nanoscale optics have numerous applications in optical technologies such as nanolithography, high-density optical data storage, photochemistry on a nanometer scale, solar cells, materials imaging and surface modification with subwavelength lateral resolution, local linear and nonlinear spectroscopy of biological and solid-state structures, quantum computing and quantum communication.
In general, solar energy can be harvested in two ways: as electricity (solar photovoltaic) and as heat (solar thermal). All extant solar cells are less than 100% efficient, meaning the solar cells convert less than 100% of the incident solar energy to a usable form. At present it appears that, high efficiency solar cells can be achieved only in p-n junction photovoltaic (PV) cells with average aperture-area efficiency (AAE) of about 10-28%, and modules with average AAE of about 17%. In research-grade multijunction concentrators, efficiencies as high as about 39% have been reported. These are based on crystalline semiconductors, which are expensive. For standard crystalline silicon (c-Si) PV technology, not only is the material cost some 50% higher than that of thin film forms, but the cost for installation is high compared to flexible substrate PVs such as those made from amorphous silicon (a-Si). Inexpensive PV cells based on non-crystalline semiconductors have the following AAE's: a-Si about 12%; CdTe (cadmium telluride) about 16%; and CIS (copper indium diselenide) about 19%. See B. von Roedern, K. Zweibel, and H. S. Ullal, “The role of polycrystalline thin-film PV technologies for achieving mid-term market competitive PV modules,” 31st IEEE Photovoltaics Specialists Conference and Exhibition, Lake Buena Vista, Fla., Jan. 3-7, 2005.
The fundamental physics behind low efficiency of inexpensive cells is directly related to the difficulty in assuring simultaneously high photon absorption and charge collection efficiencies. This results from the simultaneous requirements of the light incident on a solar cell and the electrons and holes liberated by the interaction of that light with the photovoltaic semiconductor material. That is, the photovoltaic material must be thick enough to absorb the light (in the form of photons), yet thin enough to enable the liberated charge carriers (electrons and holes) to successfully migrate to the edges (top and bottom) to be harvested.
Furthermore, for a-Si-based solar cells, the stabilized efficiency is typically about 15% lower than the initial value due to light-induced metastable defect creation, known as the Staebler-Wronski effect (SWE). D. L. Staebler and C. R. Wronski, “Reversible conductivity changes in discharge-produced amorphous Si,” Appl. Phys. Lett. 31, 292-294 (1977). Reducing the thickness and corrugating the surface of the active PV layer can improve efficiency significantly, but the low carrier mobility and lifetime product and the SWE are controlled by the band tails of the localized electronic states in the semiconductors, due to structural disorder. The structural disorder is a fundamental problem for all non-crystalline materials that reduces dramatically the diffusion length of the generated carriers.
Prior art attempts to manufacture solar cells using optical rectennas have had major difficulties in achieving large-scale metallic nanostructures at low cost. Recently, multi-walled carbon nanotubes (MWCNTs) were reported to behave like optical antennas that receive and transmit visible light incident upon them. These nanostructures were shown to be highly metallic with well aligned growth orientation. MWCNTs can also be fabricated at low cost in large scale on most conductive or semiconductive substrates by the well-established plasma-enhanced chemical vapor deposition (PECVD) method without using expensive and time-consuming state-of-the-art technologies, such as electron-beam lithography, which are unscalable but still inevitably being used by most other experimental approaches in this field. Thus, there is a need in the art to create a new class of very efficient and low cost solar cells.